Because it is being asked to absorb an impact rather than apply a force, and it is not built for it.

An ordinary cylinder generates thrust along its axis and its rod bearing supports the rod against modest side load. A stopper receives a moving mass on the side of its head: the force is transverse, it arrives suddenly, and it repeats every cycle.

The consequences on a standard cylinder are a bent or scored rod, a worn rod bearing and seal, and eventually a cylinder that leaks or binds. On longer strokes the rod may deflect visibly on each impact.

A stopper cylinder is proportioned for that duty - heavier rod, a bearing designed for transverse load, and a head made for repeated contact - and it incorporates a means of absorbing the energy rather than simply resisting it.

Using a standard cylinder works for a while, which is why it is a common shortcut, and then fails in a way that looks like premature cylinder failure rather than a specification error.

By an integral hydraulic shock absorber, a sprung head, or a cushioned mechanism - depending on the energy involved.

The simplest arrangement is an elastomeric or sprung head, which deforms on contact and spreads the impact over a longer time. It is cheap and adequate for light loads at modest speed.

For heavier or faster loads, an integral hydraulic shock absorber is used: the head is mounted on a small hydraulic damper that meters oil through an orifice as it is compressed, converting the kinetic energy into heat and bringing the load to rest over a controlled distance. That gives much smoother deceleration and handles far more energy.

Some designs use a lever or rotating head that swings against a damper, which also allows the item to release forward without the cylinder fully retracting.

The selection depends on the kinetic energy - mass and velocity - and the manufacturers publish capacity in those terms rather than as a force.

From the mass, the conveyor speed and the cycle rate - with energy, not weight, being the governing quantity.

The kinetic energy to be absorbed depends on the mass and on the square of the velocity, so speed matters far more than it first appears: doubling the conveyor speed quadruples the energy the stopper must absorb.

Cycle rate matters because a shock absorber converts that energy into heat, and it must shed the heat between impacts. A stopper adequate for occasional use can overheat on a fast line.

The nature of the load matters too: a rigid item on a belt behaves differently from a pallet on rollers or a container of liquid, which continues to move internally after the container has stopped.

Provide mass, speed, items per minute, and what the item is, and let the supplier select against their energy ratings. Sizing on weight alone is the usual reason a stopper is found to be inadequate in service.

It decelerates over whatever distance the stopper allows - and that distance determines whether the product survives the process undisturbed.

A hard stop brings the item to rest almost instantly, which means very high deceleration. Contents slosh, stacked items shift, fragile products are damaged, and anything standing upright can topple.

A cushioned stopper spreads the deceleration over a stroke of some millimetres, which reduces the peak force substantially. For filled containers, stacked cases and glass, that difference is between an acceptable process and a damaged one.

So when specifying, consider what the product will tolerate, not only what the stopper will survive. A stopper that is mechanically adequate can still be wrong for the product.

Where the product is particularly sensitive, slowing the conveyor at the stop position, or using a two-stage arrangement that slows the item before stopping it, may be necessary in addition to the cushioning.

With a second stopper or a lever mechanism that holds the following item while the first is released - a singulating arrangement.

On an accumulating conveyor, items queue against the stopper under pressure from those behind. Simply retracting the stopper releases the whole queue.

Singulation uses two stopping positions a short distance apart. The lead item is held at the front stopper; the second item is held back by the rear one. To release one item, the front stopper retracts and the rear stopper holds; once the lead item has passed, the front stopper extends again and the rear one releases, letting the next item advance into the front position.

Some mechanical designs achieve the same with a single unit and a linked pair of heads, indexing automatically as the cylinder cycles.

Specify the item length and spacing when selecting a singulator, since the geometry depends on it - a unit set up for one product size will not singulate a different one.

It adds to the load the stopper must hold and must be included in the sizing - particularly on a driven roller conveyor.

When items queue behind the stopper on a powered conveyor, the drive continues to push them forward. The stopper is then holding not just the lead item but the accumulated force of the whole queue driving against it.

That static holding force can be considerable and is entirely separate from the impact energy of the arriving item. A stopper sized only for the impact may be unable to hold the queue.

Zero-pressure accumulation conveyors avoid the problem by stopping the rollers behind a detected item, so no drive force is applied to the queue - and where that is available it is much kinder to product as well as to the stopper.

State the accumulation arrangement and the maximum queue when specifying. On a conventional accumulating conveyor with a long queue, the holding force may govern the selection rather than the impact.

Head and shock absorber condition, mounting security, and rod inspection - because everything about this duty is repeated impact.

The head takes the contact and wears or deforms; it is usually replaceable, and a worn head changes the stopping position slightly, which can matter on a positioned station.

Integral shock absorbers have a finite life measured in cycles and eventually lose their damping - the symptom is a harder stop, more noise, and product disturbance that was not there before. That change is gradual, so it is worth noting the sound and behaviour when new as a reference.

Mounting bolts loosen under repeated impact more readily than under steady load, and a stopper that has shifted slightly out of position is a common cause of intermittent jams.

Inspect the rod for scoring and deflection. A bent rod on a stopper usually means the shock absorption has failed or the unit is undersized, so replacing the cylinder without addressing that repeats the failure.